1. Field of the Invention
The present invention relates to a turbo molecular pump and more particularly to a turbo molecular pump in which the working range at the low vacuum side is extended.
2. Description of the Invention
In the prior turbo molecular pump, an axial flow blade which exhibits an excellent evacuating performance in the region of the molecular flow is used. However, this type of turbo molecular pump has a disadvantage that, when the pressure has attained 10.sup.-3 to 10.sup.-2 Torr or above, the pumping speed is rapidly reduced. Consequently, in case this type of pump is used as the exhaust pump in a process of making a semiconductor with the dry etching method in which a high vacuum on the order of 10.sup.-8 to 10.sup.-6 Torr and a low vacuum 10.sup.-2 to 10.sup.-2 Torr are repeatedly employed in a short time, since the pumping speed is small in the low vacuum, the exhaust time which is required to go from the low vacuum state to the high vacuum state is long. This has precluded improvement of the throughput. To eliminate the above described disadvantage, a composite type of turbo molecular pump has been proposed in which an axial flow molecular pump as described in U.S. Pat. No. 2,918,208 and a spiral grooved molecular pump as described in U.S. Pat. No. 4,270,882 are combined. Such a pump extends the region in which the pumping speed is reduced to the low vacuum region ranging from 10.sup.-1 to 10.sup.-2 Torr. However, even in this composite pump, a sufficient pumping speed is not obtained in the region of the viscous flow above the 10.sup.-1 Torr and a turbo molecular pump having a large pumping speed up to 10 Torr has been desired.